1. Field of the Invention
The present invention relates to a television receiver or other system using a class D amplifier for audio signal amplification.
2. Description of Related Art
Various designs for assuring the performance of a class D amplifier when used alone have been developed as taught in Japan Unexamined Patent Application Publications (kokai) H5-160649, H6-303049, and H6-216664 (corresponding to U.S. Pat. No. 5,160,896). However, when a class D amplifier is used as part of a system such as a television receiver, the fundamental frequency and harmonic components of the pulse width modulation (PWM) signals generated by a conventionally configured class D amplifier create interference with surrounding circuits, thus requiring other anti-interference measures to be taken.
This problem is particularly noticeable when two or more class D amplifiers are used as audio amplifiers for viewing stereo broadcasts. In this case, the fundamental frequency components of the PWM signal from each of the class D amplifiers not only interfere with surrounding circuits, they also interfere with each other and become superposed as an interference signal on the audio output signal, resulting in an audible interference component in the audio output from the speakers.
In addition, when a class D amplifier is used for audio amplification in a television receiver, the fundamental frequency for pulse width modulation in the class D amplifier typically exceeds 100 kHz because the peak audio frequency is typically approximately 20 kHz. A harmonic frequency component therefore extends into the video frequency band. As a result, using a class D amplifier for audio amplification can also result in interference with the video output.
The methods conventionally used to resolve the problems associated with interference produced by a class D amplifier have thus been deficient. The most common method of avoiding the effects of interference from the amplifier has therefore been to individually shield each class D amplifier with a separate shield.
Even this method, however, does not eliminate the actual problem of interference in systems using one or more conventional class D amplifiers, and the above-noted problems thus remain.